1. Field of the Invention
The present invention relates to a color filter layer for a multicolor liquid crystal display device, and more particularly, to color filters formed self-aligned to stripes of transparent electrodes, with a leveling layer provided therebetween.
2. Description of the Background
Liquid crystal flat panel display devices are increasingly used because of their low consumption of electrical power and small size. Liquid crystal devices are widely used in information displays as well as various consumer products. Recently, the performance of larger liquid crystal display devices has approached that of cathode ray tubes. In addition, as the demand increases for color display of data, including images, this trend has also expanded into hand-held displays, which typically employ reflection type liquid crystal displays because they consume less power.
Liquid crystal display devices typically include, at a minimum, a layer of liquid crystals placed between a pair of parallel transparent substrates provided with alignment films, transparent electrodes, and a pair of polarizers disposed on outward surfaces of the substrates. Although plate glass sheets have mostly been used as the transparent substrate, plastic films are increasingly used as substrates in liquid crystal displays for hand-held devices, for example in cellular phones and portable pocket books. Typically, the plastic film substrates are relatively thin and of light weight, having a thickness of from 0.1 to 0.3 mm.
In order to display color, liquid crystal display devices are provided with color filters. Various methods of color filter fabrication are disclosed which include dyeing, dispersing pigments, electrodepositing, electrolyzing (or disrupting) micelles, printing and other similar methods. Using these methods, either red (R), green (G) or blue (B) filters may be formed on a transparent substrate.
These R, G and B filters are formed such that each is distributed substantially uniformly, which requires exacting positional control during the fabrication of the filters. The precision of the color filter alignment which is feasible is generally determined by the kind and the size of the substrate, for example, and the process equipment used for the fabrication. Precise alignment of plastic substrates is more difficult to attain than with glass substrates, because plastic substrates change in dimension with changes in temperature and humidity more than glass substrates. To form color filters on plastic substrates and to achieve sufficient precision for fabricating liquid crystal device with satisfactory device characteristics, it is desirable to use fabrication processes which do not require precise positional alignment, or to reduce the need: for such stringent processes as much as possible.
Color filters can be formed through electrochemical processes on a pattern of linear transparent electrodes previously prepared: on the substrate, using, for example, the above-mentioned micelle electrolysis. In addition, the layers of color filters may also be rendered electrically conductive by containing, or being admixed with conducting materials. As disclosed in Japanese Laid-Open Patent Publication No. 6-34809 (1994), the conductive color filter layers formed in this manner may serve not only as electrodes for carrying out the electrochemical process for forming the color filters, but also for driving liquid crystal devices.
Layers of color filters formed by electrochemical processes adhere rather weakly to the transparent electrode by physical adsorption. This often leads to problems such as the color layers peeling off during subsequent device fabrication processes. To obviate these problems, Japanese Laid-Open Patent Publication No. 3-61400 (1991) proposed a method in which a resin material is disposed on and penetrates into the color filter layer. This resin material is subsequently polymerized to form a polymer structure, thereby increasing the adhesion of the color filter layer.
A variety of reflection type liquid crystal devices have been developed recently, in addition to transmission type devices. It is desirable to form reflection type displays so that light incident on the display device is reflected back efficiently to an observer, and thereby provides a display which is as clear as possible. In general, therefore, black filters or a black matrix (BM) may not be used in reflection type display devices because they decrease the amount of light reflected from the display. This is in contrast with transmission type displays, in which a black matrix is commonly used to fill gaps between the RGB filters, and thereby enhances the contrast and color purity of the display.
It may be noted that the black matrix not only blocks light which bleeds through color filters, but also assists in leveling the surface of the display, which is defined by the face of color filters which are disposed towards a liquid crystal layer. Although the required surface flatness is dependent on the type of the liquid crystal display, it is typically on the order of 0.1 micron for super twisted-nematic (STN) type displays.
Surface flatness is of considerable importance for STN type displays. As noted above, conductive color filters formed by electrochemical processes may serve as electrodes for driving display devices. Therefore, it is not desirable to form a thick leveling layer on top of color filters, especially in the case of STN displays, because the thick leveling layers cause a decrease in potential voltage applied to the liquid crystal layer. Accordingly, it is desirable to form leveling layers only in gap portions between color filters, and either a thin or no leveling layer on top of the color filters.
As long as the device is fabricated on a glass substrate, patterning process steps can be employed to make this possible. Such patterning process steps may include, for example, conventional photolithography techniques using photo-curing acrylic resin for forming the leveling layer. However, this process is difficult to apply to a display device employing a plastic film substrate because of the difficulties mentioned above in precisely positioning the plastic material.
It is therefore an object of the present invention to provide a color filter layer and a fabrication method which overcome the problems noted above.
It is another object of the invention to provide a conductive color filter layer which is formed to have improved surface flatness without the need for stringent processes requiring precise positional alignment.
To achieve the foregoing and other objects, and to overcome the shortcomings discussed above, a color filter layer for a multicolor liquid crystal display device is prodded which includes a plurality of color filters. The color filters are each formed electrochemically to be electrically conductive, and are located on linear stripes of transparent electrodes, self-aligned and substantially parallel to linear stripes which are disposed on a transparent insulating substrate. In addition, a layer of transparent resin material is further provided as a leveling layer so as to at least fill the gaps between the color filters.
According to an alternative embodiment, a starting material for the transparent resin material has a resistivity value of at least of approximately 1xc3x97109 ohmxc2x7cm, and the thickness of the leveling layer is at most approximately 0.3 micron at the top of the color filter. The leveling layer is formed from a photosensitive transparent resin disposed on the color layer, and hardened by exposure to light incident:from the backside of the transparent substrate. The backside of the transparent substrate is defined as the side of the substrate opposite that of the side on which the color filter layers and leveling layers are located.
In another embodiment, a reflection type multicolor liquid crystal display device is provided having an output panel which includes a plurality of liquid crystal cells, each cell including two transparent, insulating substrates arranged in a parallel and overlapping fashion, and each with a surface bearing at least one transparent electrode and a layer of liquid crystal material, in which the liquid crystal layer is contained between the insulating substrates. Each cell is capable of transmitting light upon the application of an electric field across the layer.
The transparent electrode of this invention is a conductive color filter formed electrochemically on linear stripes of transparent electrodes self-aligned to linear stripes on the substrate. In addition, a layer of transparent resin material is further provided as a leveling layer so as to at least fill gaps between the color filters.
Color filters can be formed without stringent processes such as precise positional alignment using the fabrication process disclosed herein. In addition, a leveling layer can be formed so as to at least fill gaps between the color filters. These processes are especially useful for plastic substrates, for which changes in dimension with changes in temperature and humidity are generally larger than those of glass substrates. Thereby, satisfactory multicolor display characteristics may be achieved, especially in reflection type liquid crystal devices with plastic substrates.